In a conventional linear actuator which is used in a valve device in a pump or the like, a solenoid is used without using a magnet. However, this type of linear actuator has a problem that when its size is made smaller, its thrust remarkably decreases as its volume decreases. Further, if only for a short time, the thrust can be increased by applying a large electric current to a coil but, when continuous operation is performed under the state that a large electric current is applied to the coil, excess heat generated may cause a problem.
In order to prevent such a problem, a linear actuator has been proposed in which a movable body provided with one piece of magnet is disposed in the inner side of a ring-shaped coil (see, for example, Japanese Patent Laid-Open No. 2002-206484).
In order to discharge liquid with a high degree of accuracy, a valve element is required to be reciprocated with a high degree of accuracy. Therefore, a cylinder pump has been proposed in which rotation of a motor is transmitted to a rotational cylinder through a deceleration gear train and the rotation of the rotational cylinder is converted to reciprocating motion of a movable body through a screw mechanism (see, for example, Japanese Patent Laid-Open No. Hei 10-184534).
On the other hand, expectation for a fuel cell has been elevated as a power supply for a portable electronic device used in information society in recent years or as a power supply for coping with air pollution or global warming. Among such fuel cells, a direct methanol type fuel cell (hereinafter, referred as DMFC: Direct Methanol Fuel Cell) in which power generation is performed by directly taking out protons from methanol provides characteristics that a reformer is not required and the volume energy density is high, and thus application to a portable electronic device has been increasingly expected.
Various DMFC's have been proposed which are provided with a power generating device having a power generating part (cell), an accommodating vessel for methanol or methanol aqueous solution (hereinafter, referred to as methanol), and a liquid feed pump for feeding methanol forcibly from the accommodating vessel (see, for example, Japanese Patent Laid-Open No. 2004-71262, Japanese Patent Laid-Open No. 2004-127618, and Japanese Patent Laid-Open No. 2004-152741).
The cell includes an anode electrode (fuel electrode) having an anode collector and an anode catalyst layer, a cathode electrode (air electrode) having a cathode collector and a cathode catalyst layer, and an electrolyte membrane disposed between the anode electrode and the cathode electrode. Methanol is supplied to the anode electrode with a liquid feed pump and air is supplied to the cathode electrode with an air supply pump or a blower.
A linear actuator with one piece of magnet has, for example, following problems. When one piece of magnet is magnetized in an axial direction, the utilization efficiency of magnetic flux is reduced further more as the magnet is downsized and thus a large thrust cannot be obtained. Further, when one piece of magnet is magnetized in a radial direction, since magnetic flux is radially generated and thus a relatively large thrust can be obtained but, in this case, the magnetizing in a radial direction is difficult. Especially, when the magnet is downsized, in the case that, for example, a magnet with the dimension of its outer diameter is small and the dimension in the axial direction is long, it is extremely difficult to magnetize the magnet in a radial direction and the magnetizing device itself may not be constructed and thus its mass production becomes difficult.
Further, downsizing is difficult in a pump device in which transmission is performed through a deceleration gear train. Further, a diaphragm valve is often used in a pump device. However, when a diaphragm valve is used, its deformation cannot be controlled accurately and thus it is difficult that a constant amount is discharged with a high degree of accuracy.
On the other hand, in the above-mentioned DMFC, activity of methanol oxidation is low in the anode electrode of a cell which is a power generating part of the DMFC and thus a voltage loss occurs. Further, a voltage loss occurs in the cathode electrode. Therefore, an output obtained from one cell becomes extremely low. Accordingly, a plurality of cell is used in the DMFC to obtain a prescribed output.
When methanol is excessively supplied to the anode electrode, a so-called crossover occurs in which a part of the methanol transmits through the electrolyte membrane in an unreacted state and leaks to the cathode electrode. Since the crossover causes the electric potential of the cathode electrode to become lower, the voltage loss is occurred in the cathode electrode. Further, unreacted methanol reached to the cathode electrode is not related to power generation and reacts with oxygen to generate heat, and thus the power-generating efficiency in a cell is significantly reduced by the crossover. Accordingly, it is preferable not to supply excessive methanol to the anode electrode.
As described above, for a pump device for supplying methanol to the anode electrode of a cell, a pump device has been desired which is provided with characteristics that discharge to a plurality of cell is possible and an appropriate amount of methanol can be accurately discharged. However, the pump device having such characteristics has not been proposed.